It's not for lack of trying. GitHub are aggressively pursuing the enterprise market from a sales perspective but there's been three problems with their approach:
1. They're pushing SaaS over on-prem: enterprise services in general are trending this way, but it only really works when you're in a dominant position - like e.g. Atlassian - when you're competing it's going to leave you with less leverage with potential clients.
2. They're leaning very heavily on AI - this is a double-edged sword as it's currently selling well to clients as a pitch but anyone doing functional evaluations is looking at benchmarks & it's not really mature enough to deliver there.
3. They're iterating really slowly on their roadmap. This seems to be GitHub's (not Microsoft's) typical approach, which I'm honestly a big fan of, but when you're entering a market with existing mature players it's going to slow down sales.
I do think GitHub will ultimately take over - it'll just happen slower than expected.